Parking brake mechanisms utilizing pressurized hydraulic fluid to apply and release the brakes have been disclosed previously in U.S. Pat. No. 5,161,650 and copending U.S. Pat. Nos. 5,201,387 and 5,128,007, assigned to the same assignee and incorporated by reference. U.S. Pat. No. 5,281,007 discloses an electric solenoid operated system for effecting the parking application and release of brakes such as those disclosed in U.S. Pat. No. 5,161,650. It is highly desirable to provide a mechanically and hydraulically operated parking valve which has a high degree of reliability and which would cost less than an electrically controlled system. However, such a mechanically and hydraulically controlled parking valve must meet all legal and functional requirements. When the valve is not being used to apply or release the brakes for parking, hydraulic pressure must be released from the brake lines, but maintained within an accumulator supply line with zero leakage. When the vehicle operator selects to apply or release the parking valve, the valve must remain by itself in the apply or release position or mode until the hydraulic actuators within the vehicle brakes have completed their movements, and then must return automatically to the off or inactive position. Also, if an accumulator within the parking brake system is not charged above a predetermined hydraulic pressure, it must not be possible to effect a release of the brakes because of a legal requirement that once the brakes are released, the system must be able to reapply the brakes. Therefore, the parking valve must prevent a release of the brakes if the accumulator within the parking brake system is not charged above a predetermined pressure.